This invention relates to semiconductor laser devices and in particular to the stabilization in narrowing the spectrum linewidth, suitable for the light emitting source in coherent optical transmission systems.
Heretofore the narrowing of the emission spectrum linewidth of semiconductor laser devices with a external cavity is effected by utilizing coupling between the modes of the cavity within the laser and the external cavity. However, the condition to get this effect is very critical and it is known that widening of spectrum linewidth, wavelength jumping or output power fluctuations are caused by slight variations in injection current and temperature. An example therefor may be the one reported in an article in the preliminary reports for the national meeting of the Institute of Electronics and Communication Engineers of Japan 4, 1985, p. 22. Further, it is conceivable to control the length of the external cavity in submicron order by jointing a piezo element with the reflecting surface of the external cavity and by utilizing electro-striction as a method for varying the length of the external cavity in order to control the phase of feedback light to the internal cavity. This method is described e.g. in IEEE J. Quantum Electron. Vol. QE-16, p. 347, (1980). However, since, according to this method, optical axis alignment, mounting process, etc. need very precise techniques, this method is not practical.